Help required fixing Techtronix 2235 Power Supply

[davecham]

I'm trying to figure out why the inverter circuit is not working properly. I'm powering the inverter with 43v from an isolated bench power supply applied across C940. I have the power supply disconnected from the rest of the circuit by desoldering the links, however all output voltages are about half the expected value. According to the service manual Q946 and Q947 alternate the current from the preregulator through the primary windings of T948 which should switch at about 20KHz. It appears to be switching but the frequency is more like 70kHz. I've tested the components in the inverter and can't see anything wrong. Any idea what I should be looking for next? I am aware of the high voltage on the secondary of T948.

[George Edmonds]

Hi

I have a 2235 and two 2236’s (the 2236 is a 2235 with the addition of  a digital DMM and frequency display).

The power supply electrolytics used by Tektronix were not of the best quality and they ALL fail in time.  You can replace the 840uf ones with 1,000uf ones as Tektronix did themselves in the later scopes, use good quality 105C Japanese electrolytics such as those made by Rubycon NOT Chinese rubbish

Have you looked at the 2235 pages on TekWiki, there is quite a lot of information there regarding the repair of the power supply.

When running off your bench supply is the current taken about 2A?

G Edmonds

[davecham]

Thanks for the reply, George - I've replaced some of the caps, but all the old ones tested ok. When I'm running off the bench power supply it's only taking about 100mA, but I've disconnected everything except the HV.

[George Edmonds]

Hi Dave

Given that it is a switch mode supply it will NOT work correctly without the secondary loads.  I would suggest restoring the secondary loads and then apply a DC voltage from a bench power supply as per the service manual to check it.

G Edmonds

[davecham]

The voltages were low before I disconnected everything - I wanted to rule out any downstream problems. But I take your point that the operation of the inverter could be adversely affected by removing the secondary load.

[George Edmonds]

Hi

Almost invariably if the power supply in a Tek 2235 has an excessive load on one of the secondary supplies, especially when running from the mains, you will hear a ticking sound.

I still am of the opinion that the best way forward is to restore all of the secondary loads and run the main supply from a bench supply as is recommended in the service manual, if all is well the current drawn should be in the order of 2A.  This is a critical step as it isolates the pre regulator and leaves any possible fault to the inverter stage only or its output loads. 

From what you have said so far the prime suspects must be Q946 and Q947 or associated circuitry.

G Edmonds